1. Field of the Invention
The present invention relates to a parking brake mechanism in a duo-servo type drum brake provided with a link type shoe drive mechanism, wherein a pushing force of the brake shoe to the drum is controlled according to an anchor reaction force so that the brake can be highly efficiently and stably applied. More particularly, the present invention relates to a parking brake mechanism for obtaining a parking brake function by a very small number of parts.
2. Description of the Related Art
Conventionally, there are provided various types of drum brakes for braking a running vehicle. According to the arrangement of the brake shoes pressed against an inner circumferential face of the substantially cylindrical drum, these drum brakes can be classified into a leading trailing type, two leading type and duo-servo type.
In general, the duo-servo type drum brake includes a pair of shoes composed of a primary shoe and secondary shoe which are arranged in the drum being opposed to each other.
Concerning the primary shoe, an entry side in the advancing rotational direction of the drum is an input section, and a delivery side in the advancing rotational direction of the drum is connected with an entry side of the secondary shoe via, for example, an adjuster. On the other hand, a delivery side of the secondary shoe is contacted with an anchor section provided on a backing plate. Therefore, anchor reaction forces acting on the primary shoe and secondary shoe is received by this anchor section.
Due to the above structure, when the primary shoe and secondary shoe are expanded and pressed against the inner circumferential face of the drum, the anchor reaction force acting on the primary shoe is inputted into the entry side of the secondary shoe, so that the secondary shoe can be pressed against the inner circumferential face of the drum. Accordingly, both the primary shoe and the secondary shoe are made to act as leading shoes. Therefore, it is possible to obtain a brake force of very high gain.
Compared with a leading trailing type or a two leading type drum brake, the above duo-servo type drum brake can provide a very high brake force, and further it is easy to reduce the dimensions of the drum brake and furthermore it is easy to incorporate a parking brake into the drum brake. In this way, the above duo-servo type drum brake has a large number of advantages.
However, the above duo-servo type brake device is sensitive to a change in the coefficient of friction of the brake shoe lining. Accordingly, there is a tendency that the brake force is difficult to be stabilized. Therefore, it is required to take countermeasure for stabilizing the brake force.
In view of the above background, the present applicant has already proposed a shoe drive mechanism characterized in that: a valve for controlling hydraulic pressure is incorporated into a hydraulic wheel cylinder for expanding and contracting a pair of brake shoes, and a brake force is stabilized by controlling a supply of hydraulic pressure to the wheel cylinder according to an anchor reaction force.
However, the brake function of the brake devices incorporated into recent vehicles has been made to be highly intelligent, for example, an anti-lock brake is provided and also a traction control system is provided in the brake function of the brake devices. Further, from the viewpoint of reducing environmental pollution, electric vehicles (EV vehicles) and hybrid vehicles have been actively developed. In order to cope with the above intelligent brake function of the brake and also in order to cope with the electric vehicles, it is the most important task to drive the brake device electrically.
In order to drive the brake device electrically, for example, instead of the conventional hydraulic type wheel cylinder, an electrically driven type operating force generator, in which an electric motor and others are used, is adopted. In this case, it is impossible to use the above shoe drive mechanism in which the supply of hydraulic pressure to the hydraulic type wheel cylinder is controlled by the hydraulic control valve according to the anchor reaction force. Therefore, it becomes necessary to develop a new shoe drive mechanism adapted to the electrically driven type operating force generator, which is a new task to be accomplished when the duo-servo type brake drum device is electrically driven.
Therefore, as a shoe drive mechanism adaptable to the electrically driven type operating force generator, there is proposed a link mechanism for transmitting an output of the operating force generator to a pair of brake shoes. Concerning the conventional link mechanism, in general, the number of parts is large, and further it is necessary to incorporate an operating force generator for driving the shoe drive mechanism according to the brake operation. Accordingly, a space to be utilized between the pair of brake shoes is reduced.
Therefore, for example, when a parking brake mechanism is incorporated, if the parking brake mechanism has a complicated structure composed of a large number of parts, the manufacturing cost is increased, and further it is impossible to ensure a sufficiently large space into which the parts are incorporated. Accordingly, it becomes difficult to incorporate the parking brake mechanism into the brake device.